1. Its not an asspull. This sort of power has been hinted at and hyped since the 8 gates have been told to us, which I believe is over 500 chapters ago, so ya not an ass pull.
2. Definitely within the top 10. Technically at 8 gates he kind of dies after any fight with anyone else, but he has very little opposition in any fight.
In all seriousness, he would likely be more like top 5 with it being
Based solely on strength:
1.Madara - Is capable of using multiple tbb at the same time and what not. Easily the strongest right now.
2.Obito - Obito has his phasing and can literally only be hurt by someone with extreme prep or Kakashi+someone else
3.Gai - Has faster reactions than Minato in sage mode and can is so fast that Madara had to retreat into his ball, which Obito only did to hide from 4 TBB capable of destroying a city each. Yes, guy is this powerful.
4.Kakashi - kamui snipe. Pew Pew. The ability to use kamui on himself also allows for dimensional travel which can be extremely handy as well.
5.Itachi - yata+totsuka+amaterasu+tsukyomi+finger genjutsu+genius
Hiruzen isn't on this list for obvious reasons, he isn't worthy.